A beamline ion implanter provides an ion beam for treating a workpiece. The ion beam may be a spot beam or ribbon beam and may be distributed across the front surface of the workpiece by ion beam movement, workpiece movement, or a combination of both. A spot beam has an approximately circular or elliptical cross section, while a ribbon beam has an approximately rectangular cross section.
Turning to FIG. 1, a plan view of a beamline ion implanter 100 providing a ribbon beam 104 for treating a workpiece 110 as is known in the prior art is illustrated. The beam line implanter 100 includes an ion source 102, an extraction electrode assembly 122, a quadrupole lens 124, other beamline components known to those skilled in the art (not illustrated), and an end station 126 having a platen 112 to support a workpiece 110 for treatment by the ribbon beam 104. The end station 126 also includes additional components known to those skilled in the art. For example, the end station 126 typically includes automated workpiece handling equipment. The entire path traversed by the ion beam is evacuated during ion implantation. The beamline ion implanter 100 may also have a controller (not illustrated) to control a variety of subsystems and components thereof. Before describing operation of the conventional beamline ion implanter 100, it is helpful to define a Cartesian coordinate system where a centroid of the ribbon beam 104 defines a Z axis. An X-Y plane defined by an X and Y axis is orthogonal to the Z axis as is shown by the coordinate system of FIG. 1, with X along the wide dimension of the ribbon beam, and Y across the thin dimension.
In operation, a plasma is formed in an ion source chamber of the ion source 102 from excitation of an input feed gas. The extraction electrode assembly 122 is positioned proximate an elongated extraction aperture of the ion source chamber and biased to extract ions from the same into the well defined ribbon beam 104. In this instance, the ribbon beam 104 has a width (W) in the X direction and a height (H) in the Y direction. The quadrupole lens 124 generates a quadrupole magnetic field in a gap through which the ribbon beam 104 passes to exert forces on the ion beam that expand the width of ion beam 104 in a horizontal plane (X-Z plane) and narrow the height of the ion beam in a vertical plane (Y-Z plane).
FIG. 2 is a perspective view of the conventional quadrupole lens 124 in more detail, while FIG. 3 is an end view sketch of the same quadrupole lens 124 when looking downstream in the Z direction or the direction of travel of the ribbon beam 104. The quadrupole lens 124 includes an upper magnetic core member 302 and a lower magnetic core member 304 spaced apart to form a gap 306 through which the ribbon beam 104 can pass. A plurality of coils may be wound along the upper and lower magnetic core members 302, 304. A left bucking coil 320, a center coil 322, and a right bucking coil 324 may be wound about the upper magnetic core member 302. Similarly, a left bucking coil 326, a center coil 328, and a right bucking coil 330 may be wound about the lower magnetic core member 304. The bucking coils keep the circulating flux in the magnetic circuit at 0, to avoid saturation, and prevent a long range dipole field from spreading to other regions where it would be undesirable. The direction of current flow in the coils is illustrated by the arrows in FIG. 2 and the symbols 340, 342 of FIG. 3. The boundary conditions proximate the gap 306 provide a quadrupole field that expands the width of the ribbon beam 104 in the horizontal plane (X-Z plane) and narrows the height of the ribbon beam 104 in the vertical plane (Y-Z plane) to provide the desired aspect ratio for the subsequent beamline. The direction of current flow in those portions 370, 372 of the center coils 322, 328 proximate the ribbon beam 104 is out of the page when viewed from the perspective of FIG. 3 looking downstream in the direction of travel of the ribbon beam.
The ribbon beam 104 may be manipulated by other beam line components (not illustrated) located downstream from the quadrupole lens 124 such as a mass analyzer, angle corrector, and a deceleration lens to name only several, before striking a workpiece 110. The front surface of the workpiece 110 supported by the platen 112 defines a workpiece plane 118. The workpiece 110 may include, but not be limited to, semiconductor wafers, flat panels, solar panels, and polymer substrates. The ribbon beam 104 at the workpiece plane 118 may have a width (W) equal to or greater than the workpiece 110. The platen 112 drives the workpiece in a direction (e.g., in the Y direction) orthogonal to the long dimension of the ribbon beam 104 to distribute the ribbon beam over an entirety of the workpiece 110.
Unfortunately, in the production of ribbon beams mechanical tolerances and other uncontrolled variations in electric or magnetic fields often produce undesirable variability in the profile of the ion beam as it proceeds along the beamline or as it impacts the workpiece. One of these variables may be referred to herein as a “roll” of the beam, that is, a rotation around the principal axis (or Z axis) of the beam. For example, FIG. 4 illustrates a cross sectional view of a ribbon beam 104 at the workpiece plane 118. Those skilled in the art will recognize that the cross sectional shape of the ribbon beam may be a generally irregular shape approximating that illustrated in FIG. 4. The ribbon beam 104 of FIG. 4 has rolled about the Z axis where the left side of the ribbon beam 104 has rotated upward as indicated by arrow 402 and the right side has rotated downward as indicated by arrow 404. FIG. 5 is a two dimensional profile of a ribbon beam illustrating another example of undesired roll aberration for an actual 400 eV boron ion beam. Similarly to FIG. 4, the ribbon beam of FIG. 5 has rolled upward on the left side of the ribbon beam and downward on the right side of the ribbon beam. Other undesired roll aberrations could also be in the opposite direction where the left side of the ribbon rotates downward and the right side rotates upward.
The drawbacks of such roll aberrations can include decreased transmission down the beamline as parts of the ribbon beam may inadvertently strike different portions of the beamline ion implanter. In addition, poor control of the incident angle at which the ribbon beam strikes the wafer can result. Accordingly, there is a need in the art for an apparatus and method to overcome the above-described inadequacies and shortcomings.